Portable heart rate monitoring devices are classically composed of a processing device and an external probe (e.g. electronic stethoscope, optical measure at ear lobe, chest belt for electrocardiogram—ECG-based measurement, etc.). The use of an external probe is often considered as a reduction of the user's comfort. ECG-based pulse rate detecting devices using external electrode probes are for instance disclosed in documents U.S. Pat. Nos. 4,108,166, 6,018,677, 6,149,602 and WO 00/51680.
Various pulse rate detection systems are known in the art. Pulse rate detection devices using pressure sensitive transducers such as piezoelectric elements are for instance disclosed in documents U.S. Pat. Nos. 3,838,684, 4,195,642, 4,331,154, 5,807,267 and WO 80/00912.
More recently, measuring techniques based on so-called photoplethysmography (or PPG) have been proposed. PPG is an electro-optic technique of measuring the cardiovascular pulse wave found throughout the human body. This pulse wave is caused by the periodic pulsations of arterial blood volume and is measured by the changing optical absorption of radiant energy which this induces. The measurement system classically consists of a source of radiant energy (usually an infra-red light source), at least one detector for detecting the intensity of the radiant energy after propagation through the human body tissue and a data processing means for extracting bodily parameters such as pulse rate or oxygen concentration in the blood. Infra-red light is predominantly used since it is relatively well absorbed in blood and weakly absorbed in body tissue. Blood volume changes are therefore observed with a reasonable contrast. The principal advantage of PPG measurement resides in the fact that it is entirely non-invasive and can be applied to any blood bearing tissue, typically a finger, nail, ear lobe, nose and, in some instances, wrist.
Since light is highly scattered in tissue, a detector positioned on the surface of the skin can measure reflections (or transmissions) from a range of depths and those reflections (or transmissions) are variously absorbed depending on whether the light encounters weakly or highly absorbing tissue. Any change in blood volume will be registered by the detector at the surface since increasing (or decreasing) volume will cause more (or less) absorption. The effect will be averaged over many arteries and veins. In the absence of any blood volume changes, the signal level will be determined by the tissue type, skin type, probe positioning, static blood volume content and of course the geometry and sensitivity of the sensor itself.
PPG systems differentiate between light absorption due to blood volume and that of other fluid and tissue constituents by observation that arterial blood flow pulsates while tissue absorption remains static. As the illuminated blood flow pulsates, it alters the optical path length and therefore modulates the light absorption throughout the cardiac cycle. Non-pulsating fluids and tissues do not modulate the light but have a fixed level of absorption (assuming there is no movement).
The result of this absorption is that any light reflected from (or transmitted through) the pulsating vascular bed contains an AC component which is proportional to and synchronous with the patients plethysmographic signal. It is this modulated component which is known as the photoplethysmographic signal. This PPG signal is superimposed onto a DC level which represents the difference between incident radiant energy and the constant absorption of the tissue, blood and anything else in the optical path with constant absorption.
PPG measurement can be achieved by measurement of the intensity of radiant energy transmitted through (transmission mode systems) or reflected by (reflection mode systems) body tissue. A reflection mode system typically has much poorer signal to noise ratio, resulting from the fact that a smaller proportion of the light which is not absorbed will be reflected than transmitted. That is the reason why most of the prior art devices and systems use a detecting arrangement that is placed on the user's finger, nail, ear lobe, nose or part of the body through which light can easily be transmitted.
PPG has widely been used for measuring arterial oxygen saturation known as pulse oximetry. The technique relies on the knowledge that haemoglobin and oxyhaemoglobin absorb light to varying degrees as a function of wavelength. In particular, the absorption characteristics of red and near infrared light are inverted for the two species. It is thus possible to derive the proportion of oxyhaemoglobin and therefore the arterial oxygen saturation from a knowledge of the absorption characteristics of the arterial blood at these two wavelengths. PPG-based oximetry sensing devices employing sensors which are typically in contact with the user's finger or nail are for instance disclosed in documents U.S. Pat. No. 5,237,994, U.S. Pat. No. 5,645,060, U.S. Pat. No. 5,662,106, U.S. Pat. No. 5,934,277, U.S. Pat. No. 6,018,673, WO 99/52420, WO 99/62399 and WO 01/25802. PPG-based oximetry and heart rate detecting devices intended to be worn on or around other parts of the human body such as the wrist or ear, are also known, for instance from documents U.S. Pat. No. 5,807,267 and WO 97/14357.
One of the main problems of PPG measurement is corruption of the useful signal by ambient light and other electromagnetic radiations (so-called light artefacts) and by voluntary or involuntary subject movement (so-called motion artefacts). These artefacts lead to erroneous interpretation of PPG signals and degrade the accuracy and reliability of PPG-based algorithms for the estimation of cardiovascular parameters.
Processing of ambient light artefacts is not critical because the influence of ambient light can be measured using multiplexing techniques and the PPG signal can be restored using subtractive-type techniques. Reference can here be made to the article “Effect of motion, ambient light, and hypoperfusion on pulse oximeter function”, Trivedi N. et al., Journal of Clinical Anaesthesia, vol 9, pp. 179–183, 1997, for a description of these problems. In contrast, processing of motion artefacts is a tough task since its contribution often exceed that of the useful pulse-related signal by an order of magnitude. It is essentially caused by mechanical forces that induces changes in the optical coupling and the optical properties of the tissue. Motion artefacts are a particularly critical problem for the design of a wrist-located pulse detecting device.
Several methods have been proposed to reduce motion artefacts in PPG signals. Feature-based algorithms have been proposed to discard the corrupted segments from the signals for instance in document WO 94/22360 (corresponding to U.S. Pat. No. 5,368,026). This kind of approach allows one to reduce the occurrence of false alarms in clinical environments, but it often degrades the signals with small motion artefacts contributions. This could lead to erroneous estimation of cardiovascular parameters.
In order to circumvent this drawback, model-based noise cancelling techniques have been applied more recently for the enhancement of optical signals. Examples are for instance described in documents U.S. Pat. No. 5,490,505, WO 94/03102 and in articles “Simple photon diffusion analysis of the effects of multiple scattering on pulse oximetry”, Schmitt J., IEEE Transactions on Biomedical Engineering, vol. 38, pp. 1194–2002, December 1991, and “Noise-resistant oximetry using a synthetic reference signal”, Coetzee F. M. et al., IEEE Transactions on Biomedical Engineering, vol. 47, pp. 1018–1026, August 2000. In such approaches a reference signal of motion is recorded and a parametric model is used subsequently to retrieve motion related influences in the optical signals. Nevertheless, motion references are classically obtained by piezo-sensors or optical measures and convey therefore only incomplete or local information of motion. This degrades the performance of model-based noise cancelling techniques since they require complete and low-noise motion reference signals.